flat down kinks are easy. You pretty much just shift your weight a little bit forward while sliding like you normally would. You can kind of practice this on a rainbow box if theres one in your park. The up and down of the rainbow sort of helps with transferring your weight.
down flat is a little bit harder in my opinion. You gotta shift your weight from slightly forward to back a bit. It's not that bad but the first few times I slipped out, caught my front edge and stuff. But once you get it, its pretty simple.
Having a low, wide stance will help if you get off balance a little bit. You'll get the feel for them tho and then it'll become natural.